A Study On Operation Parameters And Decreasing Energy Consumption During Vacuum Freeze Drying

Vacuum freeze drying has a great impact upon the production of dehydrated foods because of the superior quality of the product obtained and promises continuous expansion of the number of applications. The high energy consumption and long drying time are the key technical problems.In this paper, the development of vacuum freeze drying technology is reviewed and the importance and necessity of decreasing drying time and reducing energy consumption are expounded in detail. Based on proper assumptions, a transient-state mathematical model to describe the heat and mass transfer during freeze drying is deduced. The quasi-steady state model which is simplified from the partial differential equation above, when the differential of temperature has not been take into account, is developed and analyzed. The results shows that temperature in the porous dried layer varies-as a exponential discipline whose changing scope is determined by the mass flux and a linear distribution in temperature exists across the frozen layer. The predicted drying time is found to fits the experimental results.The effects of operation on the heat and mass transfer during vacuum freeze drying are studied in the experiment. The results indicate that drying thickness, radiant temperature and pressure of drying chamber are the most three significant factors. There's hardly any relationship between freezing rate and drying time, the sublimation drying time of slow freezing material is shorter than quick freezing one, but its desorption drying time is relatively longer, so the total drying time is approximately the same. The initial freezing temperature and freezing manner has little influence on vacuum freeze drying process. Maintaining the temperature of cold trap between -40℃ and -45℃ is beneficial to save energy and shorter drying time. It is found in the experiment that material with high water content is not suitable for quick freezing, or dried products will be deformed.On the basis of test results, the effects of operation parameters such as material thickness, heating temperature and pressure of drying chamber on vacuum freeze drying are discussed. Aiming at decreasing drying time and reducing energy consumption, these parameters are optimized and optimization results agree well with actual results.The energy consumption of different equipment in vacuum freeze drier and electricity variation are measured. Analysis to the data suggests that suitable operation to compressor and vacuum pump can save energy and the actual result fits well to this conclusion.